Re: [Xen-devel] [PATCH 1/2] x86: improve NOP use for AMD CPUs

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/12/17 10:13, Jan Beulich wrote:
> For Fam10 and later AMD recommends using the "long" NOP forms. Re-write
> the present Intel code into switch() statements and add AMD logic. This
> at the same time brings us in line again with current Linux.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Overall, the logic looks to be ok.  However, it would be better for us
to default to p6_nops and switch back to k8 if we encounter those
processors.

~Andrew

>
> --- a/xen/arch/x86/alternative.c
> +++ b/xen/arch/x86/alternative.c
> @@ -85,19 +85,34 @@ static int __init mask_nmi_callback(cons
>  
>  static void __init arch_init_ideal_nops(void)
>  {
> -    /*
> -     * Due to a decoder implementation quirk, some
> -     * specific Intel CPUs actually perform better with
> -     * the "k8_nops" than with the SDM-recommended NOPs.
> -     */
> -    if ( (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) &&
> -         !(boot_cpu_data.x86 == 6 &&
> -           boot_cpu_data.x86_model >= 0x0f &&
> -           boot_cpu_data.x86_model != 0x1c &&
> -           boot_cpu_data.x86_model != 0x26 &&
> -           boot_cpu_data.x86_model != 0x27 &&
> -           boot_cpu_data.x86_model < 0x30) )
> -        ideal_nops = p6_nops;
> +    switch ( boot_cpu_data.x86_vendor )
> +    {
> +    case X86_VENDOR_INTEL:
> +        /*
> +         * Due to a decoder implementation quirk, some specific Intel CPUs
> +         * actually perform better with the "k8_nops" than with the SDM-
> +         * recommended NOPs.
> +         */
> +        if ( boot_cpu_data.x86 != 6 )
> +            ideal_nops = p6_nops;
> +        else
> +            switch ( boot_cpu_data.x86_model )
> +            {
> +            case 0x0f ... 0x1b:
> +            case 0x1d ... 0x25:
> +            case 0x28 ... 0x2f:
> +                break;
> +            default:
> +                ideal_nops = p6_nops;
> +                break;
> +            }
> +        break;
> +
> +    case X86_VENDOR_AMD:
> +        if ( boot_cpu_data.x86 > 0xf )
> +            ideal_nops = p6_nops;
> +        break;
> +    }
>  }
>  
>  /* Use this to add nops to a buffer, then text_poke the whole buffer. */
>
>
>


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